This invention relates to improvements in piezoelectric flexure devices, as well as to improvements in methods of manufacturing such devices.
A piezoelectric flexure device is a device which, owing to its piezoelectric properties, either bends in the presence of an applied electric field or, alternatively, generates a voltage in response to being mechanically bent. In its simplest form, such a device is a bilaminar structure comprising a pair of bonded strips, at least one of which exhibits piezoelectric properties. In the presence of an applied electric field, the piezoelectric strip expands or contracts and, because the strip to which it is bonded resists such expansion or contraction, the device flexes or bends.
A piezoelectric flexure device in which only one of the two bonded strips has piezoelectric properties is known as a "unimorph". However, the more common form of piezoelectric flexure devices comprises a pair of piezoelectric strips, each being uniaxially polarized. Some such devices are known as Bimorph flexure devices, Bimorph being a trademark of Vernitron Corp. When an electric field is applied across the thickness dimension of each of the piezoelectric strips, such field being aligned with the poling direction of one of the strips and opposed to the poling direction of the other strip, one strip becomes longer while the other becomes shorter. The net result is that the structure bends or flexes, the action being somewhat akin to that of the heated bimetal strip. For a better description of the structural details of such devices and their various uses, one may refer to an article by C. P. Germano, entitled "Flexure Mode Piezoelectric Transducers," IEEE Transactions on Audio and Electroacoustics, Vol, AU-19, No. 1, March 1971.
In the paper "Electromechanical Device Using PVF.sub.2 Multilayer Bimorph" by Toda and Osaka, Transactions of the IECE of Japan, Vol. E, 61, No. 7, July 1978, there is disclosed a piezoelectric flexure device in which the piezoelectric element(s) comprises several layers of a plastic film of polyvinylidene fluoride, commonly known as PVF.sub.2. The multilayered structure is desirable from the standpoing that a relatively low voltage (e.g. 300 volts) can be used to drive a relatively thick (e.g. 1 mm) flexure device. Such a multilayered structure is achieved by sharply folding the PVF.sub.2 film back upon itself, in a zig-zag fashion, to form a multitude of pleats. Prior to being folded, both sides of the PVF.sub.2 film are provided with conductive coatings which function as electrodes by which an electric field can be applied across each layer of the PVF.sub.2 film as produced by the folding operation. Also, an adhesive material is applied to the conductive coatings for the purpose of binding all of the folded layers together.
In manufacturing multilayered flexure devices of the type disclosed in the above-mentioned paper, some difficulty has been encountered in manipulating the PVF.sub.2 film to produce the desired folded structure. Further, it has been observed that the conductive coatings on the PVF.sub.2 film have a tendency to crack at each of the sharp folds in the PVF.sub.2 film. This cracking, of course, disrupts the continuity of the electrode and prevents the applied voltage from producing an electric field across each of the PVF.sub.2 layers. While some of these cracked electrodes can be readily repaired by applying a conductive paint to the exterior surface of the device, cracks which occur in the interior of the multilayered structure cannot be so easily repaired. As may be appreciated, these problems adversely affect manufacturing costs and yield.